The present invention generally concerns a document image signal processor in a document-reading machine such as a facsimile and, more particularly, a circuit for processing document image signals of gray level.
Generally, a document image signal processor, such as a facsimile, an image scanner or an OCR (Optical Character Reader) has a mechanical section for transferring a document to a desired position and an image sensor for scanning image signals of a document. According to a conventional technique of the document image signal processor, image signals of a document scanned by means of the image sensor are converted into a black or white image by comparing the gray level thereof with a fixed, invariable threshold voltage.
Referring to FIG. 1, a conventional document image signal processor preferably for use in a facsimile system is diagrammatically shown in block diagram. Regarding the operation of the conventional document image processor, a CCD (Charge Coupled Device) image sensor 13 performs main scanning of a document (not shown) through line scanning. In this way, the CCD image scanner 13 converts optical document image signals into electric document image signals. The electric document image signals converted by the CCD image sensor 13 are applied to an analog-to-digital converter (ADC) 15 coupled to the CCD image sensor 13. Then, the image signals are converted into digital data having a predetermined bit number and the digital data converted is provided to a shading correction part 17 as an image scan data of the document. The image scan data having ununiform distribution of fluorescent light is equalized (corrected) by the shading correction part 17 and thereafter applied to a hard decision logic block 19. The hard decision logic block part 19 decides whether the document image scan data is black or white by comparing the document image data, which is corrected in the shading correction part 17, with a threshold value fixed at a predetermined level.
As occasion demands, the output decision logic of the hard decision logic part 19 is transmitted in serial to a printing part (not shown) so as to print the image signals by means of a TPH (Thermal Print Head). Otherwise, the output decision logic is transmitted to a CPU (Central Processing Unit) 23 and a memory device 25 for storing the decision logic through an interface 21. The interface 21 which is coupled to the output of the hard decision logic part 19 converts the output decision logic of the hard decision logic part 19 into parallel data with a predetermined bit number and sends a DMA (direct memory access) request signal to the CPU 23. The CPU 23 receiving the DMA request signal provides the interface 21 with a response signal to the DMA request signal so as to access parallel data with predetermined bit numbers and store the parallel data the memory 25.
In this case, the hard decision logic part 19 determines whether an image signal is black or white by comparing it with the medium gray level of a fixed value, such as the Dither Pattern of Bayer. However, in the conventional document image signal processor operating in the above manner, the image signals are subject to being degraded, because the image signals are binarized into black or white with a fixed threshold of the hard decision logic part 19 even in case the luminance difference between the document image signals and background image signals of the document is relatively small.
The above-mentioned drawback of the conventional image signal processor is detailed hereinbelow with reference to FIG. 2. In the drawing, there is shown an example of the gray image signal obtained by one-dimensional scanning in direction of the arrows over a Chinese character printed on a white paper, wherein the high level is bright and the lower is dark. A group 3 (G3) facsimile system of which resolution is 200 DPI (dots per inch) is used to obtain the result. The overall gray level is graded into 16 levels. The brightness of image scan signals obtained from photo cells of the CCD image sensor may be widely distinguished into a black level below the level 6 and a white level above the level 11. It is slightly different from the human eye system in view of the manner of recognizing black and white. It will be well understood that black and white can not be clearly distinguished at an adjacent area between the gray level 5 and the levels 11, and that the black and white level corresponding thereto can be hardly distinguished from each other. For example, the black level existing between the pixel number 4 and the pixel number 6 is considerably increased, while the white level at the same position is reduced. As shown through the example data of FIG. 2, when the gap between the black and white level reaches gradually to the size of the substantial pixel, the white level is lowered and the black level is contrarily increased, thereby the two opposite levels approach the same level. It is well known from the above explained phenomenon that the level of image signal classified as the gray zone (a zone between Tmax and Tmin) is much affected by an averaging effect of photo-cell in the CCD image sensor. It should be noted that the pixels 4, 12, 14, 16 have the same gray level 9 even in the fact that the pixel 4 is substantially of the black level and other three pixels 12, 14, 16 are substantially of the white level as obvious from considering the like corresponding part of the Chinese character.
Therefore, when the document image signals are binarized by using a single fixed threshold value, the image signals are considerably degraded, compared with the original image recognized by way of 1 human eye system, because the threshold value must exist between the Tmax and Tmin to binarize the image signals. A most typical case including the above mentioned difficulty in distinguishing black and white would be a newspaper or a photograph in which the variation of brightness distribution is very considerable. In this case, distinguishing the black and white is difficult if a only single fixed threshold is employed.